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Full text of "Text Book Of Mechanical Engineering"

Alternative Fulcra.

375

shaft k, connected by spur gear with nuts / /, which turn within
the bosses m m> and thus advance the screws n n. The latter, are
connected to the ram / by crosshead q, and thus a very even
pressure is given to the water, which finally passes to the straining
cylinder j, Fig. 331, through pipe R. The pump may be worked
by hand if necessary, or the strap fork moved by hand lever s if
power be used, and a cut-off gear at / puts both straps on loose
pulley when either end of the stroke is reached. The shackles
w and v, Fig. 331, are adjusted to suit the specimen by turning
the screws uu through the worm gear T; and x is to balance the

loose gear,1 from v downwards. Enlarged views of the shackles
are given at Y, Fig. 332, to clearly shew the gripping wedges,
slightly convex on the inside and roughed like a file.

Mr. Wicksteed's alternative fulcra, as designed for Professor
Hele-Shaw, are 'shewn in Fig. 334. Fulcrum A is employed for
heavy tests, and B for lighter tests, which are thus made with a
greater degree of sensitiveness. The lever knife-edges are level,
but the support c, which can be put in or out of position by worm-
gear, is higher than support D, as seen at (2). This gives enough
clearance for vibration either at (i) or (2), and the lever takes the
position E F when changing the centres. (See App. II., p. 836.)

The Emery Machine has obtained great favour as an
instrument of precision. Professor Unwin says of a 7 5-ton
machine: ' Every half-pound of load was precisely and: instantly
measured, whatever the stress the machine was exerting.' It is